1. Field of the Invention
The present invention relates to a vehicular electronic control unit for an automobile or the like. In particular, the invention relates to an improved vehicular electronic control unit in which the accuracy is increased in a low voltage range or a particular intermediate zone for part of many analog signals by using a multi-channel A/D converter having an ordinary resolution.
2. Description of the Related Art
Vehicular electronic control units handle many analog signals. For example, analog signals are converted by two 16-channel A/D converters having a resolution of 10 bits into digital signals, which are input to a microprocessor for 16-bit or 32-bit computation.
However, there is a problem that the resolution of 10 bits cannot provide sufficient accuracy for part of the analog signals. Using high-resolution multi-channel A/D converters to solve this problem is excessive for ordinary analog signals that do not require high accuracy, and is also costly.
To solve this problem, JP-A-2000-13227 (patent document 1, paragraphs 0006-0011 and FIGS. 1–7) discloses a technique of increasing the accuracy of A/D conversion efficiently by using both of a first A/D converter covering all the voltage range and a second A/D converter covering a low voltage range for the same analog signal. Supplied with a reference voltage 5 V, for example, the first A/D converter converts an input voltage of 0 to 5 V into a digital value of 0 to 1,023. Supplied with a reference voltage 1.25 V, the second A/D converter converts an input voltage in a low voltage range of 0 to 1.25 V into a digital value of 0 to 1,023.
In the technique of patent document 1, to secure continuity between digital conversion values in the low voltage range of 0 to 1.25 V and those in the high voltage range of 1.25 to 5 V, the reference signal 1.25 V for the second A/D converter is supplied, as an input signal, to the first A/D converter and its digital conversion value (1,024×1.25/5=256) is obtained. A variation in the reference voltage 1.25 V that is the intermediate voltage for connection of the low voltage range and the high voltage range can be compensated for by monitoring the digital conversion value of the reference voltage 1.25 V.
However, patent document 1 does not refer to the issue of zero point adjustment that should be considered in the low voltage range.
On the other hand, JP-A-11-214996 (patent document 2, paragraph 0005 and FIG. 1) discloses a technique in which outputs of a preamplifier and a main amplifier that cascade-connected to it are input to a microprocessor via a multi-channel A/D converter. An input circuit of the preamplifier is equipped with an analog switch SW1 for zero point adjustment, an analog switch SW2 for gain control, and a reference voltage source for gain control. The microprocessor acquires data for zero point adjustment and data for gain control by on/off-controlling the analog switches SW1 and SW2 and reading A/D conversion values at each time point.
JP-A-10-169500 (patent document 3, paragraphs 0009 and 0010 and FIG. 1) discloses a technique for compensating for a variation in the characteristic of an exhaust gas sensor by calibrating the detection output in a fuel-cut state that provides an atmospheric environment in the exhaust gas sensor that has an oxygen pump device and an oxygen concentration cell device and in which an air-fuel ratio is detected on the basis of a signal voltage that is obtained by differentially amplifying a voltage across a current detection resistor provided in a pump current supply circuit.
The technique of patent document 1 has problems that the second A/D converter serves only as the A/D converter for handling signal voltages in the low voltage range and that one analog signal occupies inputs of three channels in total.
The technique of patent document 2 is not intended for increase in the accuracy of A/D conversion in the low voltage range. Digital conversion values of outputs of the preamplifier and the main amplifier are fixed by the resolution of the multi-channel A/D converter used.
The technique of patent document 3 is not intended for increase in the accuracy of A/D conversion but for the variation compensation of the exhaust gas sensor for detecting the air-fuel ratio.